1. Field of the Invention
The present invention relates to a method of treating an active material, a vaporizing device which utilizes such a method, and a thin-film forming apparatus which utilizes such a device.
2. Description of the Related Art
As IC complexity has advanced, attention is being focused in recent years on the technologies of forming microstructures of submicron geometries on a semiconductor wafer with excellent controllability. Particularly, in the manufacturing process of, for example, ICs or LSI devices, miniaturization of transverse dimensions of the devices increases the irregularity of pattern formation in the microstructures. Hence, in the interconnect structure forming process, excellent coating of the irregular surface of a pattern formed at a high density and complexity with a conductive film, which is an interconnection material, and embedding of a fine opening for the interconnection, having a diameter of 1 .mu.m or less, (hereinafter, the opening being referred to as a contact hole) with an interconnection material, such as Al, are required.
Interconnection materials must be those which offer excellent ohmic contact to semiconductors, allow for formation of a film having low resistivity on an industrial basis, show excellent adhesion to a silicon or silicon oxide film, allow for fine patterning, have metallurgical properties suitable to bonding, and exhibit high reliability under normal operating conditions.
Conventionally, interconnect structures for an IC or LSI device are commonly formed by sputtering using Al or an Al alloy. To form an Al or Al alloy film, a chamber of a sputtering apparatus is evacuated to 10.sup.-5 to 10.sup.-6 Pa, Ar gas having a high purity is introduced into the chamber under a pressure of 1 to 10.sup.-1 Pa, and then a d.c. voltage is applied between electrodes to generate glow discharge. Sputtering involves bombardment by accelerated Ar.sup.+ of a cathode (target) and transportation of atoms near the surface of the target material as a vapor to a Si substrate on which an anode is placed. However, since the target material is sputtered to deposit the atoms thereof on the surface of a wafer as a source substrate, the shape of the deposited film varies depending on the irregularities of the wafer surface. Particularly, in the case of a contact hole having a diameter of 1 .mu.m or less and a depth of 1 .mu.m or above, it is difficult to completely embed the contact hole by sputtering.
Thin-film forming technologies which employ CVD involve decomposition of the material gas contained in the vapor phase which is in contact with the surface of a wafer by thermal energy in order to form a desired thin film. In CVD, since a film material can be uniformly distributed over the surface of the wafer and since a thin-film deposition occurs due to decomposition of the material gas on the wafer surface, step coverage is improved as compared with sputtering. Further, CVD enables a thin film to be selectively formed on a special substance alone by utilizing differences in the decomposition reaction mechanism of the material gas with the surface of various substances, such as Si, metals or insulating materials. It is therefore possible to selectively form a conductive film in a contact hole alone by adequately selecting the type and deposition conditions of the material gas.
Thus, various studies have been made on formation of interconnecting material films by CVD, and the possibility that the film of a metal, such as Al, W or Cu, can be formed by CVD has already been confirmed.
Most of the material gases for interconnecting films are liquid at ambient temperatures and generally have a low vapor pressure. For example, the saturated vapor pressure of dimethyl aluminum hydride, used as the material gas for an Al film, is about 2 Torr at ambient temperatures, and the saturated vapor pressure of triisobutyl aluminum is 0.5 Torr. Thus, in order to introduce a liquid material gas whose vapor pressure at ambient temperatures is low into a reaction chamber of a CVD apparatus, a vaporizing device must be provided.
Conventionally, a bubbling device is used as a vaporizing device for liquid material gases. FIG. 1 is a schematic view of a bubbling device. In the bubbling device shown in FIG. 1, a carrier gas is introduced into a liquid material 92 accommodated in a bubbling vessel 91 from a carrier gas introducing pipe 93, whereby the vapor of the liquid material 92 is taken into bubbles 95 of the carrier gas. The carrier gas which contains vapor of the liquid material is supplied from a gas discharge pipe 96.
However, organic metallic compounds used as CVD material gases are generally very active chemically, and readily react with water or oxygen at ambient temperatures. Thus, when organic metallic compounds are vaporized by bubbling, water or oxygen contained in the carrier gas as impurities readily react with such an organic metallic compound and are thereby caught in the liquid. In addition, since the vapor pressure of such a reaction product is generally lower than the vapor pressure of an organic metallic compound, the reaction product is accumulated in the organic metallic compound, gradually reducing the purity thereof as bubbling advances.
When such an organic metallic compound with impurities contained therein is used in CVD, the property and deposition rate of the CVD film formed on the substrate vary markedly, thus making the formed film unstable. Hence, deterioration in purity of the organic metal compound is conventionally prevented by increasing the purity of the carrier gas. However, since the carrier gas is generally transported to the thin-film forming apparatus via a pipe from a supply facility, even if the purity of the carrier gas supplied from the supply facility is high, the purity of the carrier gas may be reduced by leakage of a small amount of gas into the carrier gas, which would occur during transportation of the carrier gas to the thin-film forming apparatus, or when an external gas component enters the carrier gas from the inner surface of a pipe, thus reducing the purity of the organic metallic compound gas.
Thus, vaporization of a very reactive liquid compound by bubbling encounters a problem involving reduction in the purity of the organic metallic compound with time.
Furthermore, in the CVD apparatus, since a film forming chamber is evacuated by a vacuum pump after the material gas has been introduced into the chamber, a flammable compound is sucked into the pump. In addition, because such a material gas has a low vapor pressure at ambient temperatures and is thus liquid, when it is sucked into the pump, it is liquefied, and remains therein. In that case, even if an inactive gas is introduced from a ballast valve, it is difficult to completely remove the material gas in the pump. Such a material gas may ignite when the pump is opened to the atmosphere for maintenance or the like.
Conventionally, when such a flammable substance is sucked into the pump, the interior of that pump must be purged slowly before maintenance using an inactive gas, and the portion of the interior of the pump where the flammable substance is considered to be lodged must be treated under an inactive gas atmosphere. Thus, safety of the pump is reduced, and the maintenance operation is troublesome.
In a conventional chemical vapor deposition apparatus which employs an organic aluminum gas, such as trimethyl aluminum (TMA) or dimethyl aluminum hydride (DMAH), a heater (hereinafter referred to as an after heater) is disposed in an exhaust mechanism to thermally decompose an unconverted gas into nontoxic substances in the following manner: EQU 2Al(CH.sub.3).sub.3 +3H.sub.2 .fwdarw.2Al+6CH.sub.4 ( 1) EQU 2AlH(CH.sub.3).sub.2 +H.sub.2 .fwdarw.2Al+4CH.sub.4 ( 2)
FIG. 2 schematically shows an exhaust system in which an after heater is disposed. In FIG. 2, reference numeral 4 denotes a reaction chamber, reference numeral 41 denotes a mechanical booster pump, reference numeral 42 denotes an after heater, and reference numeral 43 denotes a rotary pump. The final exhaust indicated by an arrow is connected to, for example, a local exhaust pipe which is lightly evacuated by, for example, a fan.
The exhaust mechanism shown in FIG. 2 has the following problems: since an unconverted gas is thermally decomposed into a metal and hydrocarbon gas, the metal attaches to the inner wall of the heater. Thus, the operation of the apparatus must be suspended periodically to remove the metal attached to the heater. When the apparatus is used for mass production, maintenance performed to remove the metal is considered to be wasteful down-time.
The chemical vapor deposition process, which employs an organic aluminum gas, uses this gas solely or in a state wherein it is mixed with a carrier gas or hydrogen serving as a reaction gas, as shown by formulas (1) and (2). When hydrogen is mixed excessively into an organic aluminum gas, it is impossible for the after heater to convert hydrogen into a safe substance, thus causing an explosive to flow into the exhaust pipe. Thus, the exhaust gas must be connected to a strong exhaust pipe completely shielded from the atmosphere, and a special diluting mechanism must be provided to discharge such an exhaust into the atmosphere.
Another problem encountered by the conventional after heater method shown in FIG. 2 is the presence of a small amount of organic metal which has not been thermally decomposed. The gas not brought into contact with the heater surface is exhausted in the form of an organic metal. Although such a gas does not explode, it may be discharged into the atmosphere as powder of alumina.